The cell contents of a primary alkaline cell typically contain an anode comprising zinc anode active material, alkaline electrolyte, a cathode comprising manganese dioxide cathode active material, and an electrolyte ion permeable separator, typically comprising a nonwoven material containing cellulosic fibers and polyvinylalcohol fibers. The anode active material comprises zinc particles admixed with zinc oxide and conventional gelling agents, such as carboxymethylcellulose or acrylic acid copolymers, and electrolyte. The gelling agent holds the zinc particles in place and in contact with each other. A conductive metal nail, known as the anode current collector, is typically inserted into the anode material in contact with the end cap which forms the cell's negative terminal. The alkaline electrolyte is typically an aqueous solution of potassium hydroxide, but other alkali solutions of sodium or lithium hydroxide may also be employed. The cathode material is typically of manganese dioxide and may include small amounts of carbon or graphite to increase conductivity. Conventional alkaline cells have solid cathodes comprising battery grade particulate manganese dioxide. Battery grade manganese dioxide as used herein refers to manganese dioxide generally having a purity of at least about 91 percent by weight. Electrolytic MnO.sub.2 (EMD) is the preferred form of manganese dioxide for alkaline cells because of its high density and since it is conveniently obtained at high purity by electrolytic methods. EMD is typically manufactured from direct electrolysis of a bath of manganese sulfate and sulfuric acid.
The cathodes of conventional Zn/MnO.sub.2 alkaline cells the manganese dioxide composition is typically between about 70 and 87 percent by weight. Particulate graphite and aqueous KOH solution (7-11 Normal) can be added to the manganese dioxide to form a cathode mixture. Such mixtures form a moist solid mix which can be fully compacted into the cell casing using plungers or other such compacting devices forming a compacted solid cathode mass in contact with the cell casing. The cathode material can be preformed into the shape of disks forming annular rings inserted into the cell in stacked arrangement and then recompacted.
Since commercial cell sizes are fixed, it has been desirable to attempt to increase the capacity, i.e., the useful service life of the cell, by increasing the surface area of the electrode active material and by packing greater amounts of the active material into the cell. This approach has practical limitations.
If the active material is packed too densely into the cell, this can reduce the rate of electrochemical reaction during discharge, in turn reducing service life. Other deleterious effects such as polarization can occur, particularly at high current drain (high power applications). Polarization limits the mobility of ions within the electrode active material and within the electrolyte, which in turn reduces service life. The contact resistance between the MnO.sub.2 cathode active material and the cell casing of an alkaline cell also reduces service life. Such contact resistance losses typically increases, particularly as the cell is discharged during high power applications (between about 0.5 and 1 watt).
There are increasing commercial demands to make primary alkaline cells better suitable for high power application. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Such devices require high current drain rates, typically pulsed drain, of between about 0.5 and 2 Amp, more usually between about 0.5 and 1.5 Amp. Correspondingly, they require operation at power demands between about 0.5 and 2 watt. Modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps are examples of such high power applications. Thus, it is desirable to provide a way of reliably increasing the useful service life of conventional primary alkaline cells particularly for cells to be used in high power applications, without significantly increasing polarization effects or otherwise adversely affecting cell performance.
The steel casing of the alkaline cell is conventionally covered with a label containing graphics and printed information about the cell and manufacturer. It has been the practice in the battery art to make the labels as thin as possible thereby providing the cell with maximum volume for electrochemically active material. Conventional cell labels are heat shrinkable, though flexible and typically formed of polyvinylchloride (PVC) or polypropylene film. However polyethylene terephthalate film (PET) or glycolmodified polyethylene terephthalate film (PETG) are also desirable. The Label is typically single or double ply, preferably single ply. The label conventionally has a thickness of less than 10 mil (250 micron), typically less than 5 mil (125 micron). The label typically has a thickness between 3 mil (75 micron) and 10 mil (250 micron), more typically between about 3 and 5 mil (75 and 125 micron). Such label is disclosed, e.g., in U.S. Pat. Nos. 4,608,323 and 5,443,668. For example, in U.S. Pat. No. 4,608,323 it is stated that the maximum thickness of a preferred jacket (2 ply label) is about 80 to 85 micron overall. It is stated that this includes about 40 micron for the outer layer, 30 micron for the inner layer and about 10-15 microns for the ink and clear adhesive therebetween). At such thickness the label provides negligible thermal insulation.
Condition indicators (on-cell testers) for the cell can be integrated under a portion of the label, for example, as illustrated in U.S. Pat. No. 5,612,151. Since such indicators are thin and applied over only a very small portion of the casing, they have negligible insulating effect on the cell.